Expert Report

Forensic Analysis: Weighing Bullet Lead Evidence (2004)

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Since the 1960s, FBI testimony in thousands of criminal cases has relied on evidence from Compositional Analysis of Bullet Lead (CABL), a forensic technique that compares the elemental composition of bullets found at a crime scene to the elemental composition of bullets found in a suspect's possession. The report assesses the scientific validity of CABL, finding that the FBI should use a different statistical analysis for the technique, and that, given variations in bullets manufacturing processes, expert witnesses should make clear the very limited conclusions that CABL results can support.

Key Messages

Although it has been demonstrated that there are a large number of different compositionally indistinguishable volumes of lead (CIVLs), there is evidence that bullets from different CIVLs can sometimes coincidentally be analytically indistinguishable.

CABL is sufficiently reliable to support testimony that bullets from the same compositionally indistinguishable volume of lead (CIVL) are more likely to be analytically indistinguishable than bullets from different CIVLs. An examiner may also testify that having CABL evidence that two bullets are analytically indistinguishable increases the probability that two bullets came from the same CIVL, versus no evidence of match status.

Compositional analysis of bullet lead data alone do not permit any definitive statement concerning the date of bullet manufacture.

Detailed patterns of distribution of ammunition are unknown, and as a result an expert should not testify as to the probability that a crime scene bullet came from the defendant. Geographic distribution data on bullets and ammunition are needed before such testimony can be given.

The available data do not support any statement that a crime bullet came from, or is likely to have come from, a particular box of ammunition, and references to boxes of ammunition in any form is seriously misleading under Federal Rule of Evidence 403.

The committee's review of the literature and discussions with manufacturers indicates that the size of a CIVL ranges from 70 lbs in a billet to 200,000 lbs in a melt. That is equivalent to 12,000 to 35 million 40-grain, .22 caliber longrifle bullets from a CIVL compared with a total of 9 billion bullets produced each year.

The current analytical technology used by the FBI inductively coupled plasma-optical emission spectroscopy (ICP-OES) is appropriate and is currently the best available technology for the application.

Variations among and within lead bullet manufacturers makes any modeling of the general manufacturing process unreliable and potentially misleading in CABL comparisons.